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Adds a micro benchmark for Member<> write performance during and outside
of GC. The test can be used to quantify worst-case scenario for the
Member<> write barrier performance. It can be extended to support
different scenarios (e.g., young generation barrier) in future.

Output on a Z840:
[ RUN      ] WriteBarrierPerfTest.MemberWritePerformance
*RESULT WriteBarrierPerfTest writes during GC: = 21781.746896101067 writes/ms
*RESULT WriteBarrierPerfTest writes outside GC: = 476190.4761904762 writes/ms
*RESULT WriteBarrierPerfTest relative speed difference: = 21.861904761904764 times
[       OK ] WriteBarrierPerfTest.MemberWritePerformance (37 ms)

Bug: 1014414
Change-Id: I565564fe3436fbe610c4724d413d4566b289b3fb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1862274Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705911}